The present invention relates to a display device, and, more particularly, to a method of arranging a plurality of transistors, which are prepared by a pseudo single crystallization technique (SELAX: Selectively Enlarging LAser X'tallization or a method similar to SELAX), for example, a pair of transistors, in a display device.
A TFT (Thin Film Transistor) type liquid crystal display module has been popularly used as a display device in a notebook type personal computer and the like. As an example of such a liquid crystal display module, there is a known display module which employs thin film transistors (TFT) formed on polysilicon.
On the other hand, there is a technique which recystallizes polysilicon or amorphous silicon in the lateral direction (direction parallel to a substrate) using laser beams, so as to increase the particle size thereof (see brochure of International Publication 97/45827 (hereinafter referred to as a patent literature 1) and Society for Information Display 2002 (SID 02) DIGEST pp. 158-161 (hereinafter referred to as a non-patent literature 1)). For example, it has been reported that, by forming thin film transistors on a semiconductor (silicon) layer, which is formed by a method described in the above-mentioned non-patent literature 1, the mobility (μ) can be enhanced about three times compared to thin film transistors formed on a usual polysilicon film.
FIG. 11 is a diagram illustrating a pseudo single crystallization method of forming polycrystalline silicon using laser beams, as described in the above-mentioned non-patent literature 1.
In this method, laser beams 4 are irradiated to a polycrystalline silicon film 2, that has been formed on a glass substrate 1, while scanning the glass substrate 1 in the direction of the arrow A shown in FIG. 11. As the laser beams 4, continuous (CW: Continuous Wave) laser beams are used.
Due to this laser beam irradiation, the temperature of the polycrystalline silicon film 2 is elevated, and, hence, the polycrystalline silicon film 2 is melted. Then, when the irradiation advances to the next position, the temperature of the polycrystalline silicon film 2 is again lowered so that the polycrystalline silicon film 2 is recrystallized, whereby a polycrystalline film is formed, in which the particles thereof are of large size, in a thin strip shape in the lateral direction.
Hereinafter, in this specification, this polycrystalline film is defined as a pseudo single crystal region 3. However, the method of forming the pseudo single crystal region 3 is not limited to the method described in the non-patent literature 1, and a polycrystalline film which has been formed by another method, which is similar to the method described in the non-patent literature 1, is also to be included in the definition of the pseudo single crystal region 3.
Since such melting and recrystallization tends to advance in a direction opposite to the scanning direction (the direction of the arrow A shown in FIG. 11) on the glass substrate 1, the crystal growth direction (the direction of an arrow B shown in FIG. 11) of the pseudo single crystal region 3 assumes a direction parallel to and opposite to the scanning direction on the glass substrate 1.
In this recrystallization, the laser beams are converted into linear beams that are elongated in the Y direction using a beam expander. The laser irradiation intensities in the X direction and the Y direction during such an operation are shown in FIG. 12 and FIG. 13. In FIG. 12 and FIG. 13, the axis of abscissas indicates the position and the axis of ordinates indicates the laser beam intensity.
The laser beam intensity distribution in the X direction exhibits a substantially Gaussian distribution, while the laser beam intensity distribution in the Y direction generates a slight difference in intensity within a crystallization range between Y1-Y2 (a range indicated by C shown in FIG. 13), and the state of crystallization is changed in response to the difference in intensity. Further, even when the distribution of the intensity of the laser beam at the position in the Y direction is fixed, there arises the following drawback.
That is, in view of the fact that it is difficult to continue the growth of crystals in the lateral direction over an extremely long region, the following case exists. That is, in the course of the formation of the pseudo single crystal region 3, the irradiation of the laser beams 4 to the polycrystalline silicon film 2 is prevented, or the intensity of the laser beams 4 is decreased, so as to stop the growth of the crystals in the lateral direction temporarily. Then, at the position spaced away from one pseudo single crystal region 3, which is already formed, the laser beams having a given intensity are again irradiated to form another pseudo single crystal region 3 at another position separate from the previous position.
Further, although the laser beams 4 have a shape which is elongated in the direction (Y direction) which crosses the scanning direction (X direction), rather than being elongated in the scanning direction, the length in the Y direction is extremely small compared to the size of the substrate, such as the glass substrate 1. Accordingly, there may be a case in which, by performing reciprocating scanning, while shifting the position in the Y direction each time the scanning reaches an end of the glass substrate 1, another pseudo single crystal region 3 is formed at a position different from the previous position.
In this manner, when the pseudo single crystal regions 3 are formed at a plurality of positions two or more times separately, there may be a case in which the state of the crystallization differs delicately for every formed pseudo single crystal region 3, or a case in which the characteristics of the thin film transistor formed on the pseudo single crystal region 3 differs for every pseudo single crystal region 3.
By forming the thin film transistor on the semiconductor layer formed by the method described in the abovementioned non-patent literature 1, it is possible to prepare a liquid crystal display module which incorporates peripheral circuits, such as drive circuits therein. In such a liquid crystal display module which incorporates the peripheral circuits therein, a reference voltage generating circuit for a digital/analogue converting circuit (DAC) incorporated in the liquid crystal display module is necessary, or a differential amplifying circuit is necessary for buffer amplifiers, which are provided to respective drain signal lines.
The differential amplifying circuit requires a pair of transistors which agree in the transistor characteristics thereof (or exhibit a small relative error in transistor characteristics, thus having a favorable matching).
However, as mentioned previously, in the above-mentioned non-patent literature 1, depending on the intensity distribution of the laser beams used for recrystallization, the pair of transistors delicately differ in the crystallized state thereof, and, hence, there arises a drawback in that the relative error in the characteristics of the pair of transistors which are formed on the recystallized silicon layer is increased.
Accordingly, there has been a case in which a thin film transistor which is formed on a semiconductor layer by the method described in the previously-mentioned non-patent literature 1 gives rise to a problem when the transistor is applied to an analogue circuit which is required to have high accuracy, such as a buffer amplifier for a drain driver, which is required to exhibit an offset voltage of several mV or the like.
Further, the occurrence of the drawbacks explained heretofore is not limited to the pseudo single crystallization technique described in the non-patent literature 1, and the drawbacks may arise in a case in which the pseudo single crystal region 3 in which the elongated strip-like crystals grow in the lateral direction is formed using other similar pseudo single crystallization techniques. This is because these cases are common with respect to irregularities of the intensity distribution of the laser beams 4 in the direction (Y direction) which intersects the scanning direction (X direction), or with respect to the irregularities of the crystallized state when the separated pseudo single crystal regions 3 are formed at the plurality of positions.
The drawbacks attributed to such causes are hardly apparent when the particle size of the polycrystal is small, since the irregularities of th characteristics are made uniform due to the presence of a large number of polycrystals in respective thin film transistors. However, when the elongated strip-like crystals are grown in the lateral direction, the number of the crystals present in respective thin film transistors becomes small, and, hence, the irregularities become apparent.
The present invention has been made to overcome the above-mentioned drawbacks of the related art, and it is an object of the present invention to provide a display device in which the irregularities of characteristics of a pair of transistors, which are formed by a pseudo single crystallizing technique and are used in a differential amplifying circuit or the like, have been reduced.
The abovementioned and other objects and novel features of the present invention will become apparent from the description provided in this specification and from the attached drawings.
A summary of representative aspects of the invention disclosed in this specification will be set forth as follows.
The present invention is directed to a display device which includes a semiconductor layer formed on a substrate and a plurality of thin film transistors having semiconductor layers, wherein a semiconductor layer includes a first pseudo single crystal region and a second pseudo single crystal region which is formed at a position separated from the first pseudo single crystal region; and, of the plurality of thin film transistors, two or more thin film transistors, which are required to exhibit small irregularities relative to each other as characteristics of the transistors, are arranged in the same pseudo single crystal region.
Further, the present invention is also directed to a display device which includes semiconductor layers formed on a substrate and having pseudo single crystal regions and a plurality of thin film transistors arranged inside of the pseudo single crystal regions, wherein in a pseudo single crystal region, the semiconductor includes crystals which are grown in an elongate strip-like shape in the direction parallel to the substrate; and, of the plurality of thin film transistors, two or more thin film transistors, which are required to exhibit small irregularities relative to each other as characteristics of the transistors, have the direction of the length of the gates of the respective thin film transistors arranged with an inclination of within ±20 degree with respect to the longitudinal direction of the strip-like grown crystals, and they are arranged such that, when channel regions of the respective thin film transistors are imaginarily extended in parallel in the growth direction of the strip-like grown crystals, at least portions of the channel regions are superposed on each other.
Further, in accordance with the present invention, the rate of the superposition of the regions is 50% or more, and preferably 80% or more.
Further, the present invention is directed to a display device which includes semiconductor layers formed on a substrate and having pseudo single crystal regions and a plurality of thin film transistors arranged inside of the pseudo single crystal regions, wherein in the pseudo single crystal region, the semiconductor includes crystals which are grown in an elongate strip-like shape in the direction parallel to the substrate; and, of the plurality of thin film transistors, two or more thin film transistors, which are required to exhibit small irregularities relative to each other as characteristics of the transistors, have the direction of the length of the gates of the respective thin film transistors arranged with an inclination of within ±20 degree with respect to the longitudinal direction of the strip-like grown crystals, and they are arranged such that th directions of currents which flow in the respective thin film transistors are aligned with each other.
Here, two or more thin film transistors, which are required to exhibit small irregularities relative to each other as characteristics of the transistors, are formed of a differential pair of transistors which constitute a differential amplifying circuit, a pair of transistors of an active load circuit which constitute a differential amplifying circuit, or a pair of transistors of an active load circuit which constitutes a differential amplifying circuit and a transistor having a gate thereof, to which an output voltage of the active load circuit is applied.
Further, two or more thin film transistors, which are required to exhibit small irregularities relative to each other as characteristics of the transistors, are formed of a pair of transistors which constitute a current mirror circuit, or a plurality of transistors which are connected in parallel to each other and equivalently constitute one transistor.